We put engineered oak down.
We glued it direct to the subfloor, and it looks good after 3 years.
In my experience (in houses) floating floors tend to develop cracks over time, whereas glued floors stay good for ages... It's a specific high flex glue, which allows the floor to expand and contract...
If using glue you don't need underlay, but even if floating I would avoid it on a boat, as it may suck in and trap moisture.
Insulating under the ply is a good idea if you can, but beware that if the ply gets wet (big spillage, plumbing failure etc) it will want to dry out from above and below. So if you have sealed it in with insulation below, it will be likely to hold moisture and rot.
We also added vent holes through the ply along the edges of the boat, with little brass vents over (one every 6'-8'). These are really useful, allowing a view into the bilge and obviously airflow. We tried to locate them near the fire and fridge, as these suck air through the bilge. Others were located under furniture or in out of the way corners.